Adaptive loop filtering (ALF) techniques have been proposed to improve coding efficiency and the quality of encoded imagery and video. These ALF techniques design spatial filters to, among other things, minimize mean-squared error (MMSE), which is often a main source of noise. One purpose of adaptive loop filters is to reduce coding artifacts such that the filtered frames will have improved quality and will serve as better references for predictive coding. To achieve the goal of reduced MMSE, spatial filters such as Wiener filters have been designed that include filter coefficients and a control map (to indicate whether to apply the filters on block basis) which may be transmitted as side information. In some cases however, the information of filter coefficients and control map may be relatively large. Thus, the trade-off in designing adaptive loop filters involves the level of quality improvement against the size (rate) of side information.
Large filter sizes with less symmetry constraints on the filter coefficients generally have improved quality, while the amount of side information (i.e., the number of coefficients to be transmitted) is large. Imposing constraints on size and symmetry could reduce such side information at the expense of reduced quality. Most commonly used shapes for these spatial filters include square-shaped spatial filters, diamond-shaped spatial filters, and circle-shaped spatial filters. These shapes have relatively similar filtering support along vertical and horizontal directions. However, smaller vertical sizes will reduce memory bandwidth, which is attractive to hardware implementers.
It has been proposed to design special filter shapes having reduced vertical size, such as rectangular-shaped spatial filters, frusto-diamond-shaped spatial filters, or disk-shaped spatial filters. In these proposals, symmetry constraints imposed on filter coefficients may include point-symmetry. Nevertheless, such constraints may not be suitable for image/video content with strong directional features, since the filter may be forced to assigned same weight on pixels crossing an edge boundary. Directional symmetric constraints leads to improved quality, however such designs often increase the number of coefficients as compared to point-symmetry.
Cross-shaped filters have been proposed which lead to reduced number of coefficients. Also it has been shown to have good performance on content with dominant vertical and horizontal features. However since cross-shape filters do not have any coefficients on positions other than on the vertical and horizontal lines, it may not adequately resolve artifacts oriented in an arbitrary direction, especially artifacts along diagonal directions.